In a wide range of applications in communications, metrology and science, it is valuable to shift the frequency (and hence wavelength) of a beam of light. Example applications of frequency shifting include:                heterodyne detection schemes as used, for example, in Doppler velocimetry.        encoding information into a light beam through frequency modulation or frequency-shift keying.        phase shifting.        tuning the interaction of light with matter.        producing moving interference patterns for use, for example, in frequency-shifting methods.        etc.        
The frequency of a laser beam can be shifted by reflection from a moving mirror (see FIG. 1). This arrangement is known as a “Doppler mirror”. U.S. Pat. No. 5,262,889 discloses apparatus for shifting the frequency of a light beam in which the light beam is reflected from a rotating helical or spiral mirror to achieve a constant frequency shift. The frequency of a laser beam can be shifted by diffraction from a moving grating. In FIG. 2 the rotation of a radially ruled grating causes light in the +1 diffraction order to be shifted to a higher frequency while light in the −1 diffraction order is shifted to a lower frequency. In these methods, the mechanical inertia of the moving components prevents rapid changes in the frequency shift. Therefore, these methods are unsuitable for modulating frequency shift with a high bandwidth.
Frequency shifting may be achieved by varying an optical delay using an electro-optic modulator. Frequency can be decreased, for example, by continuously increasing the optical delay. An electro-optic modulator can be used to modulate a frequency shift with a high bandwidth.
An acousto-optic modulator (AOM) may be used for frequency-shifting. In an AOM, light passes through a transparent medium in which a sound beam is being propagated. The light is deflected by Bragg reflection from the refractive index variations induced by the sound waves. The frequency of the diffracted light is shifted by the Doppler effect because the variations are traveling waves. The generator of the sound is usually a piezoelectric crystal vibrated by a high voltage generator that can be switched on and off or varied in intensity or frequency to modulate the intensity and frequency shift of the diffracted light. The angle through which light is deflected by an AOM depends on the frequency.
Frequency shifting can be achieved by reflecting light from a surface on which a Rayleigh surface acoustic wave (SAW) is propagating. As in the case of an AOM, the Doppler effect leads to a frequency shift of the light diffracted by the wave. The generator of the waves can be switched on and off or varied in intensity or frequency to modulate the light. For reasonable generator power levels the modulation of the surface profile is much smaller than the wavelength of light and only a small fraction of the light is diffracted. An example of this is described in U.S. Pat. No. 4,157,863.
U.S. Pat. No. 7,231,102 describes the use of electro-optic traveling-wave modulators for frequency shifting. These make use of an electrical traveling wave propagating on the surface of a material within which the light propagates. Light is diffracted from a moving refractive index grating induced in the material by the Kerr effect. The artificial crystals required for such modulators makes them too expensive for some applications.
U.S. Pat. No. 4,927,245 describes a traveling grating formed in a semiconducting optical guide by means of a moving interference pattern formed by the crossing of two laser beams of differing wavelength. The traveling grating results from variations in charge carrier density induced within the optical guide.
U.S. Pat. No. 7,113,320 describes an array of ribbons which is illuminated such that each ribbon is illuminated by light of a different optical frequency. Motion of each ribbon is used to frequency modulate the frequency of the light that falls on it. This parallel configuration enables a very high data transmission rate for fiber optic communication.
There remains a need for practical and cost-effective methods and apparatus for shifting the frequency of light. There is a particular need for such methods and apparatus capable of modulating a frequency shift with a high bandwidth.